This invention relates to a high solids ethylene-vinyl acetate dispersion and a process for producing such a dispersion by batch polymerization of a monomer mixture having vinyl acetate and ethylene, in the presence of a stabilizing system of polyvinyl alcohol without additional surfactants. The dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPaxc2x7s when measured at 65 percent solids at 25xc2x0 C. The dispersion can be dried to form a redispersible powder. Both the dispersion and powder are useful in adhesive, coating and cementitious formulations.
Ethylene-vinyl acetate dispersions, and powders produced by drying these dispersions, are widely used in adhesive, coating and cementitious formulations. High solids dispersions are of special interest due to favorable economics through increasing the reactor efficiency. High solid dispersions that will be spray dried benefit from having less water to remove, increasing the efficiency of the spray dryer in both throughput and energy savings. A high solids dispersion must have a low enough viscosity for practical use.
U.S. Pat. No. 4,921,898; 5,070,134; 5,629,370; 5,936.020; 5,939,505; and 6,001,916 all disclose ethylene-vinyl acetate dispersions having a solids level of greater than 65 percent. These dispersions are produced in a batch reactor, and using a stabilizer system of polyvinyl alcohol and a surfactant. The presence of a surfactant in the dispersion negatively affects water-resistance, spray-drying, and redispersibility of polymer powders.
EP 1067147 discloses a continuous process for the production of a high solids ethylene-vinyl acetate dispersion using low molecular weight polyvinyl alcohol as the emulsifying agent without the use of a surfactant, producing an dispersion having greater than 65 percent solids and a viscosity of 1000 to 3000 cps at 25xc2x0 C. The application cites that conventional batch processes have not been adapted to make high solids vinyl acetate/ethylene dispersions without a surfactant.
Surprisingly it has been found that an ethylene-vinyl acetate dispersion having a solids level of greater than 65 percent by weight, and a viscosity at 65 percent by weight solids of less than 5000 mPaxc2x7s., can be produced in a batch process using polyvinyl alcohol as the stabilizer, without additional surfactants.
The present invention is directed to a polymer composition comprising an ethylene-vinyl acetate polymer dispersion stabilized with polyvinyl alcohol, wherein said dispersion has a solids level of greater than 65 percent by weight, and wherein said dispersion has a bimodal particle size distribution.
The invention is also directed to a process for forming an ethylene-vinyl acetate polymer dispersion comprising polymerizing a monomer mixture comprising vinyl acetate and ethylene in a batch process, in the presence of a stabilizing system consisting of polyvinyl alcohol, to form an ethylene-vinyl acetate polymer dispersion, wherein said copolymer dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPaxc2x7s when measured at 65 percent solids at 25xc2x0 C.
The invention is further directed to process for producing a redispersible polymer powder comprising polymerizing a monomer mixture comprising vinyl acetate and ethylene in a batch process, in the presence of a stabilizing system consisting of polyvinyl alcohol, to form an ethylene-vinyl acetate polymer dispersion; and drying said polymer dispersion to form a redispersible polymer powder, wherein said copolymer dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPaxc2x7s.
The invention is also directed to the use of the high solids dispersion and redispersible powder in adhesives, coatings and cementitious formulations.
The dispersion polymer of the present invention is formed in a batch free radical polymerization of vinyl acetate, ethylene, and optionally other comonomers including functional monomers, in the presence of a polyvinyl alcohol stabilizing system. Water forms the continuous phase, with polymer particles forming the dispersed phase.
Vinyl acetate and ethylene monomers are polymerized to form the dispersion polymer. In general, the final polymer contains from 75 to 99 percent by weight of vinyl acetate, and from 1 to 25 percent by weight of ethene. Preferably the level of vinyl acetate is from 85 to 95 percent by weight and the level of ethylene incorporated is from 5 to 15 percent by weight.
In addition to vinyl acetate and ethylene, one or more other ethylenically unsaturated monomers may also be present in the monomer mixture at up to 15 percent by weight, preferably from 5 to 10 percent by weight of the total polymer solids. Examples of said comonomers include, but are not limited to, comonomers conventionally used in compositions with ethylene and vinyl esters such as acrylates and maleates, e.g. butyl acrylate, and 2-ethylhexyl acrylate. Functional monomers may also be included at up to 10 percent by weight, and preferably from 1 to 5 percent by weight. Examples of suitable functional monomers are carboxylic acids, such as acrylic, methacrylic and maleic acid as well as hydroxyl and amide functional monomers, e.g. hydroxyethylacrylate, hydroxypropylacrylate, acrylamide, N-vinyl formamide, N-vinyl acetamide and the like. Crosslinking monomers can also be present, such as N-methylol acrylamide, and the n-alkyl esters thereof.
Additionally, certain copolymerizable monomers that assist in the stability of the copolymer dispersion, e.g., vinyl sulfonic acid and 2-acrylamido-2-methylpropane sulfonic acid or their salts may be used herein as latex stabilizers. If present, these stabilizers are added in amounts of from about 0.2 to 1 percent by weight of the monomer mixture.
The initiator is any free radical initiator, or initiator system known in the art. Suitable as polymerization initiators are the water-soluble free-radical-formers generally used in emulsion polymerization, such as hydrogen peroxide, sodium persulfate, potassium persulfate and ammonium persulfate, as well as t-butyl hydroperoxide, in amounts of between 0.01 and 3 percent by weight, preferably 0.1 and 1 percent by weight based on the total amount of the polymer dispersion. They can be used alone or together with reducing agents such as sodium formaldehyde-sulfoxylate, iron-II-salts, sodium dithionite, sodium hydrogen sulfite, sodium sulfite, sodium thiosulfate, ascorbic acid, erythorbic acid as redox catalysts in amounts of 0.01 to 3 percent by weight, preferably 0.1 to 1 percent by weight, based on the total amount of the polymer dispersion. The free-radical-formers can be charged in the aqueous emulsifier solution or be added during the polymerization in doses. Oil soluble initiators such as t-butyl hydrogen peroxide are preferred.
The stabilizing system of the present invention is low molecular weight polyvinyl alcohol. The polyvinyl alcohol is preferably partially hydrolyzed polyvinyl acetate and is used in amounts of 1 to 15 percent by weight, preferably 4 to 10 percent by weight, based on the weight of the polymer solids. Generally, the degree of hydrolysis will vary from 50 to 99 percent, preferably from 80 to 99 percent of the acetate groups. The polyvinyl alcohol should also exhibit a viscosity of about 2 to 45 mPaxc2x7s., preferably 3 to 30 mPaxc2x7s, and most preferably 3 to 10 mPaxc2x7s for a 4 weight percent aqueous solution at 20xc2x0 C. as determined by the Hoeppler falling ball method. Exemplary of the polyvinyl alcohol component include AIRVOL A205, a low molecular weight, 87 to 89 percent hydrolyzed polyvinyl acetate; and AIRVOL A203, a low molecular weight, 87 to 89 percent hydrolyzed polyvinyl acetate, which are marketed by Air Products Corporation. Blends of various polyvinyl alcohols may also be used in order to increase the water resistance of the resultant powder and can include 92 to 99 percent hydrolyzed polyvinylalcohol, such as AIRVOL Al 03 from Air Products or RS 105 from Kuraray.
The stabilizing system of the present invention could optionally include protective colloids in addition to the polyvinyl alcohol. Examples of useful colloids include, but are not limited to, polyethylene glycol, cellulosics, and polyvinyl pyrrolidone. The stabilizing system is free of surfactants.
The polymerization process is a batch process, involving a single reactor with all monomer added prior to commencing the reaction. In general, the process includes charging the reactor initially with vinyl acetate, ethylene, water, polyvinyl alcohol and any other suitable components. This initial charge represents 100 percent of the total monomer charge. The ingredients may be added in any order without affecting the resultant dispersion. The reactor is then heated to from 40 to 60xc2x0 C., preferably about 50xc2x0 C. The reactor is agitated by any suitable means to facilitate dissolution of the ethylene. A portion of the initiator is added to the initial charge, with the remainder added gradually during the reaction to maintain the reaction. Generally the reaction will last several hours, preferably up to 10 hours and most preferably from 1 to 4 hours.
The polymerization is carried out at a pH of between 2 and 7, preferably between 3 and 5. In order to maintain the pH range, it may be useful to work in the presence of customary buffer systems, for example, in the presence of alkali metal acetates, alkali metal carbonates, alkali metal phosphates. Polymerization regulators, including mercaptans such as mercaptoacetic acid and mercaptoethanol; aldehydes; chloroform; methylene chloride and trichloroethylene, may also be added.
It has been found that a seeded process is beneficial, but not essential. A seeded process involves the addition of from 0.01 to 10 percent by weight of a seed polymer, based on the weight of the final dispersion, to the reactor charge. Preferably the seed is an ethylene vinyl acetate latex polymer stabilized with polyvinyl alcohol, as known in the art. Most preferably the seed has a fine unimodal particle size distribution. An example of such a polymer is VINAMUL V3265, by National Starch and Chemical.
The reaction is generally continued until the residual monomer content is below about 1 percent The reaction product is cooled slightly and further initiator is added in order to reduce the residual monomer below 1000 ppm. The completed reaction product is then allowed to cool to about room temperature, while sealed from the atmosphere. After degassing, the pH may then be suitably adjusted to ensure maximum stability. Other adjustments or additions may optionally be made at this time, as desired.
The dispersion produced has a high solids level, without the need for an additional concentration step. By high solids, as used herein, is meant that the polymer particles are present in the dispersion at a level of 65 percent by weight or greater, preferably 67 percent by weight or greater, and most preferably greater than 68 percent, based on the dispersion.
The high solids dispersion of the present invention has a viscosity of a level to facilitate transport and also useful in a spray dryer, without further dilution. Preferably the viscosity of the high-solids dispersion is less than 5,000 mPaxc2x7s., and most preferably less than 3,000 mPas, when measured at 65 percent solids at 25xc2x0 C.
Dispersions formed from the present invention have a bimodal particle size distribution. While not being bound by any theory, it is believed that the bimodal distribution may be due to the formation of both emulsion and suspension polymers with the reaction system. Particle sizes of the dispersion particles from the process are illustrated in FIG. 1, and consist of a peak with a maximum between 0.2 and 0.5 microns, and a peak with a maximum between 3 and 6 microns.
The dispersion formed in the present invention may be used in the aqueous form, or may be dried to form a redispersible powder. Drying is done by any means known in the art, such as freeze drying, drum drying, fluidized bed, or spray drying. A preferred method is by spray drying under conditions known in the art. These conditions are illustrated in the Examples. The high solids level of the dispersions is advantageous in the drying procedure, since less water must be removed, requiring less time and expense.
Since the dispersions of the present invention are stabilized without the use of surfactants, water-resistance properties are not adversely affected. This makes these dispersions, or powders formed from the dispersions with excellent water resistant properties. The absence of a surfactant also improves both the spray-drying, and the redispersibility of the powder.
Typical applications for the dispersion include, but are not limited to, to self-leveling floor screeds, ceramic tile adhesives, packaging and converting adhesive applications, and coatings. Typical applications for the powder include, but are not limited to adhesives, cement additives, and cementitious, gypsum based, gypsum-free, and cement-free mortars. These mortars may contain ingredients, such as for example: quartz sand, calcium and magnesium carbonates, silicates, cellulose, calcium oxide, other minerals, or mixtures thereof